The present invention is especially directed to differential pressure thermoforming machines of the type wherein cup-shaped articles are formed in a web of thermoplastic material. Specific examples of machines of the type to which the present invention may be applied, are those disclosed in U.S. Pat. Nos. 3,496,257, 3,172,159, and 4,255,382.
Thermoforming machines of the type with which the present invention are concerned typically have opposed male and female mold or matched die members which are vertically reciprocated cyclically between the mold open and mold closed position. When the two molds are in their mold open position, an associated web feeding mechanism feeds a web of thermoplastic material between the two molds, while the molds are in their open position. Typically, one of the female molds is formed with a cavity, or a series of mold cavities, having associated vacuum ports, and the male mold member is formed with a plug assist, or a series of projecting plug assist members, which, as the molds are moved toward their mold closed position, press the aligned portions of the heated and deformable thermoplastic web into the respective cavities in the female mold.
In order to achieve uniform thickness in the finished article, it is customary to clamp the web against the face of the female mold around the periphery of the area in which the mold cavities are located, and around each cavity, at a point in the cycle before the plug assist members begin to deform the sheet. With such a clamping action, deformation of the sheet is confined to that portion of the sheet within the clamp periphery.
Because the clamping action must take place before, or at the time, the plug assist members engage the thermoplastic web, it is common practice in this field to mount the clamping ring device upon the male mold member so that it is engaged to clamp the web by the initial movement of the male mold from its open position toward its closed position. Because the male mold must continue to move toward the female mold after the clamping ring device is engaged, the usual practice is to mount the clamping ring device upon the male mold either by springs, or fluid pressure cylinders which act as springs, to accomodate the necessary relative movement between the clamping ring device and male mold after the clamping ring device has been engaged with the web to clamp it against the female mold. The subsequent relative motion compresses the springs or fluid in the fluid cylinders substantially as the molds are being closed and, upon closure of the molds it is necessary that the molds be held in their closed position for the period of time necessary to complete the forming operation. Thus, a substantial amount of power is required to overcome the spring forces urging the mold open throughout the forming operation.
The present invention is especially directed to an arrangement in which a clamping ring device of the type described above is mounted for movement entirely independently of the molds, and is driven to and from, and maintained in its clamping position, by a cam means which does not require the continued application of power to hold the clamp in its clamped position during the forming operation.